Page 177 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Fabric and Structure
172 Geotechnical Engineering

8.3.3 Shear-Induced Orientation of Clay Particles

Shearing of clay, either in a landslide or as a result of horizontal pressures in
expansive clays, creates shiny surfaces that indicate that the clay particles have
become oriented parallel with those surfaces. These ‘‘slickensides’’ are perma-
nently weak contact surfaces.

The goal of compaction is to push soil grains together, but if compaction is
continued too long or if the soil is too wet, compaction pressure is transferred to
the soil water that has no resistance to shearing. As a result the soil shears, or slips
internally, reorienting and smearing clay particles along the shear planes. Such a
soil is said to be overcompacted, and is permanently damaged by shear planes and
slickensides. Before it can be reused and recompacted it must be dried and
pulverized to destroy the sheared structure.

8.3.4 Pedologenic Structures

As mentioned in Chapter 5, clay-rich B horizons often develop a blocky structure
from a three-dimensional array of tension cracks developed from seasonal drying
shrinkage. This structure is shown in Fig. 8.6 and with increasing depth is
transitional to a columnar structure between vertically oriented shrinkage cracks.
Under low pressures the blocky structure can cause clay to behave more like
a granular soil than a clay. The blocky structure is preserved by shiny oriented

Figure 8.6
A blocky ‘‘B
horizon’’ structure
that is transitional
to a columnar
structure with
depth. Soil above
the dotted line is
fill. The soil color
is a mottled gray
and brown color
indicating wet
conditions prior to
exposure by
erosion and
sliding.

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